I upgraded to Gnome3 this week, and after half a day of debugging I got
my (quite non-standard) setup working completely again. One of the
things that got broken was my custom wallpaper on the gdm3 login screen.
This used to be configured in /etc/gdm3/greeter.gconf.defaults, but
apparently Gnome3 replaced gconf by this new "gsettings" thingy.

Anyway, to change the desktop background in gdm, add the following lines
to /etc/gdm3/greeter.gsettings:

For reference, I also found some other method, which looks a lot
more complicated. I suspect it also doesn't work in Debian, which runs
gdm as root, not as a separate "gdm" user. Systems that do use such a
user might need the more complicated method, I guess (which probably
ends up storing the settings somewhere in the homedir of the gdm
user...).